1. Technical Field
The present disclosure generally relates to electronic circuits. It more specifically relates to power dimmers intended to control a load (for example, a lighting device) powered from an A.C. voltage (generally, the mains voltage of the electric distribution network).
2. Discussion of the Related Art
Among known power dimmers, some use a triac in series with the load, the assembly receiving the A.C. power supply voltage, and a microcontroller for controlling the triac.
To power the microcontroller, the dimmer generally comprises a circuit providing a D.C. voltage across a storage capacitor. This circuit, for example, comprises a diode for rectifying the A.C. voltage to charge the capacitor, and a zener diode for setting the value of the D.C. voltage across the capacitor.
In operation, the triac is turned on at an intermediate stage between the beginning and the end of each positive and/or negative halfwave of the power supply voltage, by application of a start pulse on its gate. Once started, the triac remains on until the end of the ongoing halfwave. By varying the time of application of the start pulse, it is possible to regulate the power supplied to the load. In particular, a starting at the beginning of a halfwave of the power supply voltage amounts to supplying more power than a starting at the end of a halfwave.
One of the issues with the use of a triac lies in the fact that switchings are relatively abrupt, and are thus capable of causing unwanted current and/or voltage gradients (for example, current and/or voltage peaks) across the triac or the load. To avoid the propagation of such gradients on the electric power distribution network, it is necessary to provide an LC filter between the triac and terminals of connection to the dimmer network, which increases the bulk and the cost of the dimmer.
The LC filter may further result in malfunctions of the dimmer. Indeed, if the LC filter attenuates the amplitude of parasitic disturbances in switchings of the triac, it causes as in return, for a short time after each switching of the triac, local oscillations of the current flowing through the triac. Such oscillations, when they occur after a switching from the off state to the on state, may carry on for a time longer than the time of application of the turn-on pulse to the triac gate. There then is a risk that the current flowing through the triac will temporarily cancel due to the oscillations, thus causing a blocking of the triac almost immediately after its starting (triac uncoupling). The load then no longer receives the prescribed power during the considered halfwave.
Another disadvantage of the LC filter is that the inductance may emit an unwanted acoustic noise (crackling) each time it is submitted to a significant current variation, that is, on each starting of the triac.
Another problem of dimmers using a triac or another power component controlled by a microprocessor has to do with the microcontroller power supply. In the case of a dimmer using a triac, at the beginning of each halfwave (positive and/or negative) of the A.C. power supply voltage, although no current flows through the triac (since it has not been started yet), a current however flows through the dimmer, pulled by the microcontroller power supply circuit to recharge the storage capacitor. This current is not negligible since on each starting of the triac, the microcontroller should apply a current of relatively high value, for example, on the order of 5 mA or more, on the triac gate. The microcontroller power supply circuit and the storage capacitor should be sized accordingly. An issue is that for low power consumption loads, for example certain light-emitting diode lamps or certain low-power compact fluorescent lamps (for example, with a power lower than 10 watts), the microcontroller power supply current sampled by the dimmer at the beginning of each halfwave of the mains power supply voltage may turn on the load. As a result, in practice, dimmers using a triac generally cannot be used to control low-power loads. In the case where a power component other than a triac is controlled by the microcontroller, the microcontroller power supply current is not necessarily pulled at the beginning of a halfwave, but the same issue remains.